This invention relates to the transfer of full-color photographic images to substrates, to adhesive-coated photographic paper especially adapted therefor, and to fade- and abrasion-resistant colored structures made thereby.
It is often desirable to enhance the aesthetic value of photographic works by using special techniques to simulate an oil painting on canvas. Several methods have addressed this problem, albeit with only limited success. For example, a photographic emulsion containing silver halide particles can be knife-coated or otherwise applied to a canvas under controlled light conditions, exposed to imaging radiation through a photographic negative, and the thus-exposed emulsion layer subjected to developing solutions to yield a photographic image. The irregular surface of the canvas substrate tends to cause optical distortion during exposure, and the developing solutions tend to cause shrinkage of the substrate during processing. Further, the thin and fragile emulsion layer remains unprotected, and unreinforced, necessitating the application of a lacquer coating thereover to impart durability. Even more significantly, this procedure is limited to black and white images, primarily because of the extreme difficulty in applying the precisely positioned and proportioned multiple layers of emulsion required to produce a chromatically faithful colored photographic image.
For many years photo resists for platemaking in the printing and graphic arts industry were composed on clear film or glass "flats" by adhering photographic images carried on relatively thin "permanent supports" which had been stripped from relatively thick "temporary supports" after developing; see, e.g., U.S. Pat. No. 3,282,643. These "stripping films" were classified as either "wet" or "dry", depending on the technique employed to separate the emulsion-carrying permanent support from the temporary support. U.S. Pat. No. 3,359,107 describes a stripping film having a permanent support layer which is preferably (or always) opaque white and relatively thick (25-30 micrometers).
By and large, the physical characteristics deemed essential for a stripping film drastically limit its satisfactory performance in the transfer of a colored photographic image to an irregular substrate. An extremely thin permanent support (which is desired in a stripping film) causes extreme difficulty in handling and consequent distortion, especially when a large sheet is involved. On the other hand, stretchiness (which can not be tolerated in a stripping film, where the stripped film is to be adhered to a smooth surface) is highly desirable where the stripped film is to be conformed to an irregular surface, such as canvas. A thick film minimizes the distortion problem and simplifies handling but it causes optical distortion and edge effects in the composed piece, especially where individual colored emulsion layers are superimposed. In any event, stripping films commonly use high contrast black and white emulsion or photopolymer systems rather than multiple color silver halide emulsion layers.
A currently commercial full color emulsion transfer process utilizes a "wet" stripping film. The several emulsion layers making up a full color silver halide emulsion are coated on a solvent-soluble temporary support, exposed to imaging radiation and developed; the colored emulsion image is then separated by dissolving the temporary support. The extremely delicate and unsupported emulsion image is then carefully transferred to a second surface, to which it is adhered. Since this procedure requires handling an unsupported emulsion, its practical utility is limited to those who specialize in this process.
Because of the limitations inherent in the preceding processes, other techniques have been explored in an attempt to achieve a full-color photograph on a textured substrate. For example, three photographic emulsion layers (respectively sensitive to blue, green and red), each optionally carried by its own extremely thin individual support film, may be sequentially coated on a conventional backing. The multi-layer emulsion is thereafter exposed to imaging radiation, developed and each emulsion layer in turn (on its support film, if such is present) transferred to the permanent support, making certain that the three imaged layers are in register; see, e.g., U.S. Pat. No. 2,182,814. Great care is required in handling the individual emulsion layers, and it is extremely difficult to obtain high quality results, especially if the substrate is irregular or textured.
A somewhat analogous approach is disclosed in U.S. Pat. No. 3,721,557, where individual transparent positive images of different primary colors in a photopolymer carried by a stripping layer are in turn superposed and adhered together in register on a desired substrate. Not only is it impossible to obtain full gray scale contrast or a full range of colors with photopolymers, but the several stripping layers and adhesive layers also tend to shift the color rendition unless each such layer is water-white.
Prior to the present invention, the most popular way of generating a colored photographic image on a canvas or other irregular substrate involved a process known as "`RC` photo paper stripping". This process utilizes a curl-resistant commercially available photographic print paper of the type where a relatively thick white paper has a thin polyethylene foil laminated to each major face, the multiple photographic emulsion layers being coated over the first foil surface, which normally contains white pigment. A colored photographic image is developed in the emulsion layer and the exposed surface of the second foil adhered to a supporting substrate. Exercising great care, the paper backing is then split as close to the paper-contacting face of the emulsion-coated first polyethylene foil as possible, the first foil and developed composite image being carefully removed by rolling them around a small cylindrical dowel. The exposed face of the removed polyethylene foil, bearing the developed emulsion layers on the other face, is then adhered to the canvas with a suitable heat-activated or liquid resin and subjected to pressure to conform it intimately thereto.
The results obtained by the splitting process just described are somewhat unpredictable, even though the adhesion of the polyethylene foil to the backing can be controlled; see, e.g., U.S. Pat. No. 4,237,206. The removed emulsion-bearing first polyethylene foil can retain a significant and inconsistent amount of paper fiber torn from the original base, often resulting in an irregular caliper; this in turn may impart a distorted and irregular appearance when the curled, fragile, and unsupported foil is thereafter manipulated during the mounting process. Some have addressed the handling problem by bonding a relatively thick transparent heat-activated protective film to the emulsion side of the photo print prior to stripping, the protective film overlying and protecting the imaged surface after the polyethylene supporting foil has been adhered to the canvas substrate. While use of the protective film makes it easier to handle the emulsion layer, an extra processing step is required, the time, and the added thickness of the protective film detracts from the original objective of achieving a true canvas texture in the finished work. As in all prior art colored photographic prints, fading occurs upon extended exposure to light.
In summary, prior to the present invention, there has been no simple, convenient way to achieve a faithful color rendition on an irregular substrate. Even in those situations where such a rendition has been achieved, the colors were subject to fading.